Med-Tech Innovation Materials
It is worth noting that a device can fail at much lower stresses in use than those shown in Figure 2, for example, 20% of the maximum values, under cyclic fatigue conditions. The failure is not due to the stress being too high, but rather the flaw size increasing with time under cyclic stress. Catastrophic failure suddenly occurs when the flaw/defect grows and reaches a critical size6 and the critical value of fracture toughness of that material, governed by the law of fracture mechanics: KIC = σ(πa)
[1]
Figure 2: Fracture stress as function of materials’ flaw/defect sizes for given fracture toughness KIC (MPa.m1/2)
The basic principle worth bearing in mind is that fracture toughness is a material constant (for a given condition/environment). All variegations observed in fracture toughness are actually variations of fracture stresses and of flaw/ defect sizes that exist in the finished products or develop later during use.
Development of toughened ceramics Ceramics are well known brittle materials. However, most recently ceramics have been used as hip components with success. This is an important breakthrough after the use of metals and later polymers as implant materials. Ceramics have several advantages over metals and polymers. They are the most chemically and biologically inert of all materials. They are also strong and hard. To date, almost all reported results demonstrate that ceramics produce the lowest rates of wear particles compared with metal and/or polyethylene.7–15
is their fragility. Unlike metals and polymers, ceramic materials cannot deform under stress. When the stress acting on medical ceramic materials exceeds a certain limit, the material breaks. Therefore, toughening ceramic will be a challenging task, but an important one for materials scientists when developing orthopaedic materials of the future. There are several ways to do this. One of them is to use one ceramic to toughen another ceramic and another way is to make ceramic polymer hybrids.
Figure 3: Specimens to measure fracture toughness; sample (1) is the ideal sharp crack and sample (2) is the machined blunted crack
Figure 4: Microscopy of zirconia toughened alumina (bright phase = zirconia and dark phase = alumina)
Ceramic toughening ceramic Many types of ceramics are potentially applicable for the development of materials for orthopaedic applications. To date, alumina and zirconia- toughened alumina have been explored. One important parameter to evaluate a ceramic as good or bad is the fracture toughness KIC. However, it is a difficult parameter to measure with great accuracy and with confidence. For example, the three-point bending test is commonly used to measure fracture toughness. However, it is difficult to make an ideal pre-crack with sharp ending tip in a ceramic before testing, as shown in Figure 3, specimen (1). The basic theory of fracture toughness is based on that sharp crack. Instead of a sharp crack, a three-point bending specimen is machined to produce a notch with a blunt end, as shown in Figure 3, specimen (2) for the measurement of fracture toughness.16
It is obvious that, for the same crack size a, the two
specimens in Figure 3 will give rise to very different fracture toughness values. In addition, the quality of the tool and the speed to produce the notch will each have an influence on the reproducibility of the measured KIC results, and will seriously affect accuracy. It is important to accurately measure fracture toughness KIC for the development of new ceramics for orthopaedic applications. The fracture stress and maximum defect size allowed can be accurately predicted by using Equation [1]. The investigation of testing methods of fracture toughness and other mechanical properties precisely and accurately will be discussed in a separate article. The following gives one example of zirconia-toughening alumina to explain the new concept regarding design and development of these kinds of materials. Figure 4 shows the microstructure of one material developed using
Figure 5: Fracture surface of (a) alumina control and (b) zirconia toughened alumina
zirconia to toughen alumina (ZTA), where bright phase of the microscopy is zirconia and the dark continue phase alumina. Figure 5 shows a typical fracture surface microscopy for ZTA and the control. It is clearly shown that the control is more brittle than the ZTA because the former has less deformed features than the latter. This is further confirmed by fracture toughness KIC
22 ¦ April 2011
www.med-techinnovation.com
However, the main disadvantage of medical ceramic materials
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54